LOS ANGELES (AP) — ESPN ended its collaboration with PBS on an investigation of the NFL and players' head injuries as public TV producers expressed surprise over the abrupt collapse of the 15-month partnership.

ESPN said its decision was based on a lack of editorial control over "League of Denial: The NFL's Concussion Crisis," airing in October on PBS' "Frontline" public affairs series. At ESPN's request, its logo was being removed from websites related to the project and from the film itself.

"Because ESPN is neither producing nor exercising editorial control over the 'Frontline' documentaries, there will be no co-branding involving ESPN on the documentaries or their marketing materials," ESPN said in a statement. "The use of ESPN's marks could incorrectly imply that we have editorial control."

It was a mistake on ESPN's part that it didn't reach the conclusion sooner, ESPN spokesman Josh Krulewitz said Friday.

The head injury issue has potentially vast implications for the NFL, with more than 4,000 former players suing the league over claims it hid known concussion risks, leading to high rates of dementia, depression and even suicides.

Some believe the players' claims could be worth $1 billion or more if they move forward in court. The cases involve the deaths of players, medical care of players with disabling dementia, and lifelong medical monitoring for those who are now symptom-free.

The NFL on Friday denied a New York Times report that it had pressured ESPN to drop out of the project with "Frontline." The sports network has a lucrative contract to carry league games on "Monday Night Football."

The Walt Disney Co.-owned ESPN pays the NFL more than $1 billion a year for the broadcast rights, the Times said.

ESPN denied that the NFL had any influence.

"The decision to remove our branding was not a result of concerns about our separate business relationship with the NFL. As we have in the past, including as recently as Sunday, we will continue to cover the concussion story aggressively through our own reporting," the channel said in a release.

In a separate statement, ESPN President John Skipper defended the channel as a leader "in reporting on the concussion issue, dating back to the mid-1990s," and said he wanted to stress its commitment to journalism and support for the work of its reporters.

Raney Aronson, "Frontline" deputy executive producer, said she and others at "Frontline" were taken aback by ESPN's decision and that they weren't privy to details of why it was made.

"It's anybody's guess right now about what actually happened," said Aronson.

In an online statement, she and "Frontline" executive producer David Fanning said they regretted ESPN's exit after a productive editorial partnership with ESPN's investigative program "Outside the Lines."

The Sunday report cited Friday by ESPN as an example of its continuing reporting on concussions was part of its partnership with "Frontline," which Aronson noted was credited on the program.

ESPN executives were long aware of the "Frontline" approach to "League of Denial," Aronson said, with the PBS series controlling what it aired or posted online and ESPN doing the same for its programs or postings.

"Frontline" had been working closely with ESPN's senior vice president and news director Vince Doria and senior producer Dwayne Bray, with no indication of discord until last Friday, she said.

The two-part "League of Denial," airing Oct. 8 and 15, draws on reporting by Steve Fainaru and Mark Fainaru Wada, ESPN reporters and brothers who have a forthcoming book on the subject, and original "Frontline" reporting.

The authors and their work will remain part of the documentary, said Aronson and Fanning.

"The film is still being edited and has not been seen by ESPN news executives, although we were on schedule to share it with them for their editorial input," the producers said, adding that the two-part documentary will meet the "rigorous" standards of fairness, accuracy and depth practiced by "Frontline."

At a Television Critics Association news conference earlier this month, Bray was queried about ESPN's work on the film given its business relationship with the NFL and responded by calling ESPN a "bifurcated company."

"You do have the business partners on one side, but you also have the editorial production side," Bray said, adding that "Frontline" is the "gold standard" of long-form investigative documentaries and ESPN is the same for sports journalism.

"So we respect Frontline greatly. They respect us. And the NFL is going to have to understand that," he said.

The NFL calls player safety a top priority and insists injury claims should be handled through league arbitration, in accord with the collective bargaining agreement.

The league has instituted safety measures that include rules changes designed to eliminate hits to the head and neck, protect defenseless players, and prevent concussed players from playing or practicing until they are fully recovered.